Most of the analog broadcasting television signals are interlaced, such as NTSC, PAL or SECAM. In the case of NTSC, 262.5 scan lines (one field) are presented every one-sixtieth of a second, which is half of each frame. The interlaced video has advantages for delivering acceptable video image without exceeding broadcasting bandwidth limitations. However, it has disadvantages of lack vertical resolution, area flicker and line flicker etc. Hence, so called line doubling or de-interlacing techniques are developed to convert interlaced television signal (NTSC, PAL or SACAM) to a non-interlaced or progressively scanned television signal.
A frame (in the case of NTSC, 525 lines) rather than a field is scanned in 60 Hz. Theoretically vertical resolution is doubled to the full frame resolution 525 lines. Especially in the case of using inversing pull down method, progressive oriented content video source is converted back to frame image, which are in turn converted to interlaced video using 2-3 pull down or 2-2 pull down approach from 24 Hz or 25 Hz progressive film picture. Those conversions are very frequently used in the applications such as progressive televisions, television standard converters, television signal processors, and video signal compressors.
Typically, line doublers employ field merge or inter-field interpolation to recover the full frame resolution. Generally speaking, merge is used in static region of the image and inter- or intra-field interpolation are used in motion part of the picture. In so doing, however, a common problem called “jaggies” occurs. This problem is also called diagonal-edge or saw-tooth effects. Specifically, these artifacts appear on both stationary and moving diagonal edges as a jagged saw-toothed pattern.
Another serious problem happens in inverse 2-3 pull down processing, which merges the fields from originally same film frame. When the television is in PIP (picture in picture) mode, a small video overlaps on a film image. The solution for this case generally is to quit film merge operation and perform regular interpolation. But before changing over from one mode to another, the saw-tooth pattern appears on the video region.